NHS England said it was "deeply regrettable" that thousands more operations have been cancelled.

The walkout, which will still see junior doctors provide emergency care cover, is the fourth round of industrial action taken by the British Medical Association (BMA) in the ongoing dispute over a new contract for junior doctors.

The number of operations cancelled for both inpatients and planned day cases now stands at 24,577.

The news comes as the Royal College of Physicians of Edinburgh (RCPE) said it had written to the Health Select Committee asking it to conduct and inquiry into the "escalating crisis" relating to the contract.

Professor Derek Bell, president of the RCPE, said: "The situation should not have escalated to this level as matters of this importance can only be resolved in a sustainable way through negotiation.

"The college believes that an urgent inquiry by the Health Select Committee could help resolve the current impasse and restore stability within the junior doctor workforce in England.

"If we do not resolve this dispute, the impact on our patients, the NHS workforce and the long-term sustainability of the NHS will be profound."

Junior doctors are objecting to a new contract in England which the Government says will create a truly seven-day service.

They are currently paid more for working unsocial hours at night or at the weekend. But under the proposed new contracts, the Saturday day shift will be paid at a normal rate in return for a rise in basic pay.

The dispute has become increasingly fraught and junior doctors have two strikes planned for this month.

The first will be a 48-hour strike starting at 8am on Wednesday April 6, with junior doctors providing emergency care only.

But strikes planned for April 26 and April 27 will see the full withdrawal of labour by junior doctors - everyone up to consultant level - between the hours of 8am and 5pm on both days.

Commenting on the action, Dr Anne Rainsberry, national incident director for NHS England, said: "We've already seen that a 48-hour strike puts considerably more pressure on the NHS and it's deeply regrettable that thousands of patients are still facing disruption because of this recurring action.

"As always, the safety and care of patients is our number one priority and everything possible is being done to make sure patients will still be able to access urgent and emergency services.

"Following closely on from the four-day Easter break, this will be a difficult period especially over the course of the second day. Consequently we have redoubled our planning efforts and will be closely monitoring events to make sure we can respond to any rising pressures."

A Department of Health spokeswoman added: "This strike is irresponsible and disproportionate, and with almost 25,000 operations cancelled so far, it is patients who are suffering.

"If the BMA had agreed to negotiate on Saturday pay, as they promised to do through Acas in November, we'd have a negotiated agreement by now. We ask doctors to look at the detail of the contract and call on the BMA to cancel their plans to escalate strike action even further."

Yesterday it emerged that Health Secretary Jeremy Hunt is facing a second legal challenge to try to block the imposition of the contract, with the threat of action from NHS staff campaign group Just Health.

The BMA announced last week it was launching a judicial review challenging the lawfulness of the imposition of the contract.